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What are the differences between steam and water vapor, and what is the best term to use to describe the white cloud of water droplets above a boiling kettle?
Question Date: 2019-10-11
Answer 1:

Steam and water vapor are both used to describe water in the gaseous state. Sometimes, we use these terms interchangeably because they are describing the same concept. There is a subtle difference, though, which we can understand by thinking about different forms of water in the gaseous state. When you boil water, you see water in the gaseous state coming off of the liquid water still in the kettle. We know from experience that this gaseous water is hot. The term for this form of water is steam. On the other hand, we can also see gaseous water in the form of clouds or fog. The term for this form of water is usually water vapor.

The difference between steam and water vapor is the difference in temperature of the water molecules. You would not be wrong to call the gaseous water coming off a boiling kettle "water vapor," but saying "steam" lets us know that this gaseous water is in a vapor state and is hot. Water in the gaseous state can always be called "water vapor," and we can think of "steam" as a type of water vapor--hot water vapor.

Answer 2:

This is a classic "rectangles-squares" question. All steam is water vapor, but not all water vapor is steam. Water vapor just means the gas form of water. Water is a molecule, not specifically a liquid. It is a liquid at room temperature, and we need it in liquid form to live, so we often use the word "water" to mean "liquid water," but really water can be a solid (ice), liquid (what we call plain water), or gas (water vapor). Steam is the specific case where the water is a gas because it is at or above its boiling point (100 C). An example of water vapor that is not steam would be humidity- this is just water that diffuses into the air due to something called vapor-liquid equilibrium, which is basically the tendency for any liquid to give off some gas molecules, even if it is not boiling.

When a kettle boils- it produces steam because the boiling process is what causes the water to turn to gas. The droplets you are describing, however, are liquid water. As the steam cools down, it once again turns back into a liquid.

Answer 3:

Steam and water vapor, scientifically speaking, both refer to water molecules in the gas phase, in other words, the water molecules that freely float in the air. However, we usually use the word "steam" to refer to the visible condensation of water molecules in the air, while we most often use the term "water vapor" to refer to the invisible water molecules above a body of liquid water. Because of these conventions, we often use the word steam to describe the white cloud above a boiling kettle.

Answer 4:

Steam and water vapor are the same thing.

Technically, the white cloud of water droplets is exactly that: a cloud, and the process that creates it is exactly the same as the process that creates clouds in the sky. People often call it 'steam' because they form when steam comes into contact with cooler air and condenses to form droplets, but technically, it's a misuse of the word 'steam'.

Answer 5:

Water vapor is water as a gas, where individual water molecules are in the air, separate from each other. Steam is what you see above a boiling kettle. Steam is hot water in droplets almost big enough to see - but you can see the cloud of droplets.

There must be stages in between, where a few water molecules are sticking to each other, but the droplet is too small to see. If it's a hot day and the relative humidity is near 100%, I'm guessing there will be water molecules that stick to each other, at least briefly.

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